Regulatory response of intramembranous absorption of amniotic fluid to infusion of exogenous fluid in sheep

Six fetal sheep were operated on at 118 to 121 days of gestation. The pulmonary end of the trachea was connected to the gastric end of the esophagus with a section of tubing. This left urine as the only source of amniotic fluid and intramembranous absorption as sole exit. Multiple indwelling fetal vascular, intra-amniotic, allantoic, and a fetal bladder catheter were placed. Beginning 5 days after surgery, all urine was drained from the bladder and immediately reinfused into the amniotic sac to monitor urine production rate. After 4 days of urine infusion alone, the urine infusion was augmented for 6 days with an intra-amniotic infusion of Ringer solution. Amniotic and allantoic fluid volumes were measured at autopsy. During the period of Ringer infusion, intramembranous absorption of amniotic fluid increased by more than 1,192 ± 186 (SE) ml/day (P < 0.002) and the rates of Na⁺ and Cl^- absorption increased to more than five times (P < 0.005) and eight times (P < 0.005) their initial values. Only one of six fetuses had polyhydramnios. It is concluded that intramembranous absorption of amniotic fluid makes a strong regulatory adjustment in response to an abnormal increase in inflow of exogenous fluid.